Investigators found no clear warning signs that the children would resort to suicide, McGinnis said.
He said the the 11-year-old girl was a gifted student whose grades inexplicably sunk to Ds this semester. Still, she was well-liked by her peers and had penned a positive poem about herself on her computer the day she died.
"It was a very nice little poem; reading it, you would think this was a child with good self-esteem," McGinnis said.
The boy had been kept home from school as punishment for bad behavior the day he took his life, according to McGinnis.
His mother banished him to his room with orders to clean it; when she checked on him later, he was dead. Yet the boy was excited about attending a party that night, McGinnis said. Further, he had no apparent problems at school or with his peers.
"I have 38 years on the job," McGinnis said. "When the 11-year-old happened, I had never seen that. And then four days later, the 9-year-old. What are the odds it would happen in one week? This is tragic, heartbreaking; it keeps you awake at night."
In the most recent cases, the two kids didn't know each other, went to different schools and lived in different neighborhoods. Investigators don't believe the second suicide was a copycat case.
Police turned the kids' computers over to the FBI for some cyber-sleuthing, in hopes of finding clues in chat rooms, e-mail or other computer activity, McGinnis said. Neither child left a suicide note.
Although children have killed themselves accidentally while playing the "Choking Game," Gulino has ruled these three cases to be suicide.
Such games - also called the Passout Game, Good Kids' High, Blackout or Gasp - involve depriving the brain of oxygen with the goal of getting a quick high when blood flow returns. Kids do it by wrapping their throats with belts or ropes or compressing each other's chest until they hyperventilate and pass out.
"In the cases that we've had, we're not seeing any clear indication of [such games]," Gulino said.
Experts agree suicide among young adolescents is rare, affecting only 1.3 of every 100,000 kids ages 10 to 14 in 2006, according to the federal Centers for Disease Control and Prevention.
But for young people ages 15 to 24, suicide is the third leading cause of death, according to the CDC.
The recent suicide "cluster" prompted Gulino to examine Philadelphia's cases in hopes of uncovering trends that could better direct prevention efforts.
Since 2003, when his office's current computer system was installed, 63 youths under age 18 have killed themselves, Gulino said.
Thirty-six did it by hanging, the most common suicide method in that age group, Gulino said. Those cases occurred in four loose clusters in the Northwest, Northeast, North-Central and South parts of the city, he said.
"About 70 percent [of the hangings] were 12- to 16-year-olds; that seems to be the target group, if we're talking about what we can do for prevention," Gulino said.
Half took place between March and June, Gulino noted.
"We're dealing with relatively small numbers, so you don't want to overinterpret things," he said. Still, the timing suggests "anxiety that tends to come with the transitions that come with the end of the school year."
Suicides overall in Philadelphia have ranged from a low of 144 in 2006 to a high of 199 in 2008, according to Gulino's office. Firearms are the most common method, used in 40 percent of city suicides.
McGinnis said police already have visited schools in his division to counsel students against suicide. But he urges parents to be vigilant.
"Suicide in my mind is a permanent solution to a temporary problem. It's beyond belief that young kids would even think about it," McGinnis said. "It's up to the parents to teach the children that all problems are temporary; there's life after the problem."